In an optical disk reproduction system, a photodiode senses light transmitted to a disk by an optical pickup and converts the sensed, reflected optical signal into an analog RF signal. The analog RF signal output of the photodiode is provided to an asymmetry correcting apparatus, which corrects the asymmetry of the RF signal and converts the RF signal into a digital eight-to-fourteen modulation (EFM) signal. Namely, the asymmetry correcting apparatus slices the analog RF signal on the basis of an asymmetry-corrected slice reference level, and obtains the digital EFM signal. An example of such an asymmetry correcting apparatus is provided in xe2x80x98EFM comparator (CXA12710)xe2x80x99 (hereinafter, referred to as a conventional asymmetry correcting apparatus) described at page 89 of the xe2x80x9cSONY SEMICONDUCTOR IC DATABOOKxe2x80x9d published in 1990 by SONY(copyright).
Hereinafter, the structure and operation of the conventional asymmetry correcting apparatus will be described with reference to the attached drawings.
FIG. 1 is a circuit diagram of a conventional asymmetry correcting apparatus. The apparatus includes capacitors C1, C2, and C3, a comparator 10, operational amplifiers 12 and 14, resistors R1, R2, R3, R4, R5, and R6, and inverters I1 and I2, with Vcc signifying a power supply.
The operational amplifier 14 shown in FIG. 1 operates as an automatic asymmetry buffer. Resistors R1, R2, R3, and R4 and the operational amplifier 12 operates as an automatic asymmetry control amplifier. A low pass filter (LPF) 16, which can be connected externally, for detecting a direct current (DC) offset, is comprised of resistors R5 and R6 and capacitors C2 and C3. A CMOS buffer 18 is comprised of inverters I1 and I2.
Capacitor C1 shown in FIG. 1 eliminates the direct current component of the RF signal input via input terminal IN1, from an optical diode (not shown). The comparator 10 compares the alternating current (AC) component of the RF signal input at the positive input terminal thereof with a reference signal output from the operational amplifier 12, and outputs the comparison result through an output terminal OUT, as a digital EFM signal. The digital EFM signal is buffered by the CMOS buffer 18, and passes through the low pass filter 16, the asymmetry buffer 14, and the automatic asymmetry control amplifier 12, and is input to the negative input terminal of the comparator 10 as a reference signal.
A disk defect such as a scratch or hole is caused where the material of the disk is so poor, or the disk is so deeply gouged, that light passes through the disk without being reflected. Accordingly, the data is not detected. In the above-mentioned conventional asymmetry correcting apparatus, when the optical disk has such a defect, it can take considerable time, for example the time constant of the slice reference level, which is the level of the reference signal output from the operational amplifier 12, to determine the middle level of the RF signal. For example, since the time constant of the low pass filter 16, including resistors R5 and R6 and capacitors C2 and C3 is, for example, 5 ms, it is difficult for the slice reference level to track the middle level of the RF signal in the portion of the signal having a scratch or hole defect, which is much shorter than 5 ms in duration. Therefore, since the asymmetry of the RF signal is not properly corrected, systems which depend on an asymmetry-corrected signal, for example an error correction circuit are likely to malfunction.
The present invention relates to an optical disk reproduction system, and more particularly, to an apparatus and a method for correcting asymmetry in an RF signal output generated by an optical diode in the optical disk reproduction system, in a manner which overcomes the limitations of the prior art.
It is a first object of the present invention to provide an open-loop type apparatus for correcting asymmetry in an optical disk reproduction system, which can correct asymmetry using an envelope of an RF signal.
It is a second object of the present invention to provide a method for correcting asymmetry in an optical disk reproduction system using an envelope of an RF signal, in an open-loop system.
It is a third object of the present invention to provide an apparatus for correcting asymmetry in an optical disk reproduction system, which can correct asymmetry using a slice reference level obtained by subtracting the alternating current component of an RF signal from the RF signal.
It is a fourth object of the present invention to provide an asymmetry correcting method, in optical disk reproduction system using a slice reference level obtained by subtracting the alternating current component of an RF signal from the RF signal.
It is a fifth object of the present invention to provide a closed-loop type apparatus for correcting asymmetry in an optical disk reproduction system, which can correct asymmetry using an envelope of the RF signal.
It is a sixth object of the present invention to provide a method for correcting asymmetry, in optical disk reproduction system using an envelope of the RF signal, in a closed-loop system.
Accordingly, in a first embodiment, the present invention comprises an asymmetry correcting apparatus in an optical disk reproduction system, for correcting asymmetry in an RF signal received from an optical diode and for outputting the asymmetry-corrected signal as a digital EFM signal. The apparatus comprises a first envelope detector for detecting an upper envelope of the RF signal, a second envelope-detector for detecting a lower envelope of the RF signal, a level controller for controlling the level of the sum of the upper envelope and the lower envelope and outputting a signal having the controlled level, and a comparator for comparing the controlled level with the level of the RF signal, and outputting the comparison result as the digital EFM signal.
To achieve the second object, a second embodiment of the present invention comprises an asymmetry correcting method in an optical disk reproduction system, for correcting asymmetry in an RF signal received from an optical diode and for obtaining a digital EFM signal. The method comprises the steps of detecting an upper envelope and a lower envelope of the RF signal, adding the upper envelope to the lower envelope, obtaining a slice reference level by controlling the level of the addition result, determining whether the slice reference level is less than the level of the RF signal, determining a first logic level to be the level of the digital EFM signal when the level of the RF signal is larger than the slice reference level, determining a second logic level supplementary to the first logic level to be the level of the digital EFM signal when the level of the RF signal is less than the slice reference level, and changing the level of the digital EFM signal when the slice reference level is the same as the level of the RF signal.
To achieve the third object, a third embodiment of the present invention comprises an asymmetry correcting apparatus in an optical disk reproduction system, for correcting asymmetry in an RF signal received from an optical diode and for outputting the asymmetry corrected signal as a digital EFM signal. The apparatus comprises a capacitor for removing the direct current component of the RF signal, a signal subtracter for subtracting the capacitor output from the RF signal and for outputting a signal having a level equal to the subtraction result, and a comparator for comparing the level of the signal output from the signal subtracter with the level of the RF signal, and outputting the comparison result as the digital EFM signal.
To achieve the fourth object, a fourth embodiment of the present invention comprises an asymmetry correcting method in an optical disk reproduction system, for correcting asymmetry in an RF signal received from an optical diode and for obtaining a digital EFM signal. The method comprises the steps of obtaining an alternating current component of the RF signal by removing a direct current component of the RF signal, obtaining a slice reference level of the RF signal by subtracting the alternating current component from the RF signal, determining whether the slice reference level is larger than the level of the RF signal, determining a first logic level to be the level of the digital EFM signal when the slice reference level is larger than the level of the RF signal, determining a second logic level supplementary to the first logic level to be the level of the digital EFM signal when the slice reference level is less than the level of the RF signal, and changing the level of the digital EFM signal when the slice reference level is the same as the level of the RF signal.
To achieve the fifth object, a fifth embodiment of the present invention comprises an asymmetry correcting apparatus in an optical disk reproduction system, for correcting asymmetry in an RF signal output from an optical diode and outputting the asymmetry corrected signal as a digital EFM signal. The apparatus comprises a first envelope detector for detecting an upper envelope of the RF signal, a second envelope detector for detecting a lower envelope of the RF signal, a level controller for controlling the level of the sum of the upper envelope and the lower envelope and outputting a first reference signal having a level equal to the level-controlled sum, a first low pass filter for filtering out a high frequency component of the first reference signal and outputting the filtered result, a second low pass filter for filtering out a high frequency component of the digital EFM signal and outputting the filtered result, an amplifier for amplifying a signal output from the second low pass filter by a predetermined amount and outputting the amplification result, a subtracter for subtracting the output of the first low pass filter from the amplifier and outputting the subtraction result, an adder for adding the output of the subtracter to the first reference signal and outputting the addition result, and a comparator for comparing the output of the adder received as a second reference signal with the RF signal, and outputting the comparison result as the digital EFM signal.
To achieve the sixth object, a sixth embodiment of the present invention comprises an asymmetry correcting method in an optical disk reproduction system, for obtaining a digital EFM signal by correcting the asymmetry of an RF signal output from an optical diode. The method comprises the steps of detecting an upper envelope and a lower envelope of the RF signal, adding the upper envelope to the lower envelope, obtaining a reference signal by controlling the level of the addition result, extracting a direct current component of the reference signal, subtracting the direct current component of the reference signal from a direct current component of the previously asymmetry-corrected digital EFM signal, obtaining a slice reference level by adding the subtraction result to the reference signal, determining whether the level of the RF signal is larger than the slice reference level, determining a first logic level to be the level of the currently asymmetry-corrected digital EFM signal when the level of the RF signal is larger than the slice reference level, determining a second logic level supplementary to the first logic level to be the currently asymmetry-corrected level of the digital EFM signal when the level of the RF signal is less than the slice reference level, and changing the level of the digital EFM signal when the slice reference level is the same as the level of the RF signal.